1. Field of the Invention
The present invention relates to a ceramic heater, particularly to a ceramic heater effectively applied to a glow plug of a diesel engine, etc.
2. Description of the Prior Art
The glow plug has been used for improving the startabillity of the diesel engine at a low temperature. So, the glow plug is required to exhibit a rapidly heating property.
The present inventors have developed a ceramic heater for use in the glow plug, which is formed by integrally sintering a heater element made of a mixed powder of molybdenum disilicate(MoSi.sub.2) as an electrically conductive ceramic having excellent oxidization resistance, and silicon nitride(Si.sub.3 N.sub.4) as an electrically insulating ceramic having low coefficient of thermal expansion, and a heater support member formed of electrically insulating ceramic sintered body for supporting the heater element, top end of which is covered with the heater element (U.S. application No. 659,959 now U.S. Pat. No. 556,780 and Japanese Patent application No. 59-70670), which formed the basis for U.S. application Ser. No. 06/717,875. The glow plug provided with this ceramic heater is superior in the rapidly heating property since the heater element thereof directly heats the interior of the combustion chamber.
MoSi.sub.2, which is a constituent of the heater element gives oxidization resistance to the heater element while Si.sub.3 N.sub.4 gives thermal shock resistance to the heater element.
However, this ceramic heater has a problem that thermal stress is produced in the joint of the support member and the heater element due to the difference in various properties such as the coefficient of thermal expansion, the thermal conductivity, etc. so as to decrease the thermal shock resistance of the ceramic heater.